Group III nitride compounds and their alloys, including GaN, AlN, InN, AlGaN and InGaN, have been developed for various optoelectronic and electronic applications. Gallium nitride (GaN) has received increased attention for use in optoelectronic and electronic semiconductor devices. GaN materials including AlGaN, InGaN and InAlGaN may have desired material characteristics and open the door to new devices and applications including solar cells, high-speed electronics, and short-wavelength and long-wavelength light emitters such as ultraviolet (UV), violet, blue, and green, yellow, red and infrared light emitting devices. While GaN materials and related devices may hold promise for new devices and applications, growth of bulk GaN crystals for use as substrates in such devices and applications has presented a number of challenges. One major limitation is the inability to fabricate single crystal bulk GaN, AlN, and InN materials at high growth rates with desired material qualities.
For example, it is known to grow low defect single crystal GaN materials using Metal Organic Chemical Vapor Deposition (MOCVD). With MOCVD, group III nitride compounds are grown from the vapor phase using metal organic compounds as sources of the Group III metals. Trimethylindium (TMI) is typically used as an indium source material, trimethylaluminum (TMA) is used as an aluminum source material and trimethylgallium (TMG) is used as a gallium source material. Ammonia gas is typically used as a nitrogen source.
These materials are supplied to a MOCVD reactor from source tanks that are located outside of the reactor. Within the MOCVD reactor, a metal organic material source reacts with ammonia resulting in the deposition of an epitaxial layer of a group III nitride material on a substrate. Electrically active impurities are introduced into the MOCVD reactor during material growth to control the electrical conductivity of the grown materials. More specifically, undoped group III nitride compounds normally exhibit n-type conductivity. Donor impurities, such as silicon or germanium, can be introduced into the grown material to control n-type conductivity and form materials with low electrical resistivity. Magnesium impurities in the form of metal organic compounds are introduced into the MOCVD reactor to form nitride materials having p-type conductivity. For example, Publication No. WO 00/68470 to Solomon et al. describes a MOCVD epitaxial growth system that includes a bubbler containing a magnesium-containing compound cyclopentadienylmagnesium (Cp2Mg). Hydrogen gas is used to carry the magnesium containing compound from the Cp2Mg source into the reaction zone to form materials having p-type conductivity.
While MOCVD has been used with some effectiveness in the past for growth of certain materials, MOCVD has limited applicability for growth of bulk group III nitride materials such as GaN, including p-type GaN. A very low growth rate is a major limitation of MOCVD. For example, with MOCVD, GaN can be grown at a rate that does not exceed several microns per hour. Consequently, deposition of GaN crystals having thicknesses on the order of millimeters is not feasible using MOCVD.
In addition to growth rate limitations, MOCVD systems have other shortcomings. For example, the configuration described by Solomon et al. is not suitable for HCl etching of magnesium delivery tubes in order to remove any magnesium-containing deposits from the tube walls. Magnesium deposits may result in inconsistent doping, which is not desirable, particularly for use in applications that require consistent doping characteristics including various high speed communication electronics, light emitting diode and laser diode devices.
Hydride vapor phase epitaxy (HVPE) has also been investigated as an alternative to MOCVD for fabricating group III nitride materials including p-type GaN and other materials. HVPE offers a number of advantages over MOCVD and other fabrication techniques including materials having low defect densities, improved growth rates, controllable doping, less complicated equipment and reduced fabrication costs. Further, HVPE growth can be performed at atmospheric pressure, thereby eliminating the need for vacuum equipment. HVPE is also suitable for mass production of semiconductor materials, structures and devices due to its low cost, excellent material characteristics, flexibility of growth conditions, and reproducibility. Examples of known HVPE reactors and methods for epitaxial and bulk growth of group III nitride materials, including p-type group III materials, are described in U.S. Pat. Nos. 6,447,604 to Flynn et al.; 6,596,079 to Vaudo et al., 6,613,143 to Melnik et al., 6,616,757 to Melnik et al., 6,656,285 to Melnik et al. and 6,936,357 to Melnik et al., “Vertical-HVPE as a Production Method for Free-Standing GaN-Substrates” by B. Schineller et al. (2007), and “Growth of AlN films and their characterization” by R. Jain et al., the contents of all of which are incorporated herein by reference.
It is known that defect densities of group III nitride layers grown on foreign substrates such as sapphire rapidly decrease as the thickness of the grown layer increases. For this purpose, HVPE can be used to grow thick layers or boules of such materials, e.g., having thicknesses of about 10 to 100 microns and thicker, to provide higher quality materials and devices with reduced defect densities. This may be accomplished by growth of materials at high growth rates and/or with long growth cycles. Long growth cycles may be an option with the consequence that more time is required. Further, challenges exist in efficiently fabricating thicker material or boules at fast growth rates (e.g., greater than 1 mm per hour) with desired material quality and low defect densities.
For example, Vaudo et al. describe growth of group III-V nitride boules at growth rates in excess of 50 microns (0.05 mm) per hour, with growth rates of 200 microns (0.2 mm) per hour being preferred, and growth rates in excess of 500 microns (0.5 mm) per hour being most preferred. To the inventor's knowledge, while Vaudo et al. may generally state that is preferred to have growth rates in excess of 500 microns per hour, such capabilities have not been implemented or are difficult to implement while achieving desired material qualities. For example, as noted by Schineller et al., as GaN layers grown by HVPE become thicker, a transition in the surface morphology is observed and presents challenges in growing GaN material with desired thicknesses and material qualities. For example, Schineller et al. explain that growth of GaN in a vertical HVPE reactor at rates only as high as 400 microns (0.4 mm) per hour were achieved. Further, as noted by Jain et al., HVPE as been successfully utilized to grow free-standing GaN substrates with growth rates only as high as 200 microns (0.2 mm) per hour, citing a Journal of Crystal Growth article by Vaudo et al. (2002).
More particularly, single crystal GaN material degradation at high deposition rates is due, in part, to resulting parasitic deposition of GaN on internal components of a HVPE reactor during growth of single crystal GaN on a seed upon which the single crystal material is grown. Parasitic deposition degrades reactor components, shifts growth parameters and may even lead to a crash of the HVPE reactor. More particularly, parasitic deposition on reactor components may stress the reactor components leading to cracking and damage of the components. Additionally, parasitic deposition may result in growth of inclusions in the GaN crystal itself. Thus, although a HVPE reactor may be configured for single crystal growth, the resulting material may have inclusions. Material degradation may also result from non-stable growth rates that occur during extended periods of growth and non-uniform supplies of source materials over growing surfaces.
Thus, known HVPE reactors and methods have been utilized to achieve GaN growth at rates up to 0.2 mm to 0.4 mm per hour. However, to the inventors' knowledge, known HVPE reactors and growth methods have not been successfully employed to grow high quality, uniform and low defect density GaN bulk single crystals at higher growth rates, e.g., greater than 1 millimeter per hour, due in part to parasitic deposition at these high growth rates. To increase the growth rate, it is necessary to increase amount of source materials in the growth zone which, in turn, increases the amount of source material in the growth apparatus, which leads to an increase in parasitic reactions and depositions.
A further limitation of known HVPE reactors and methods is growth of sufficiently thick p-type group III nitride materials having desired material qualities at high growth rates. Examples of known HVPE reactors and methods for fabricating p-type group III nitride materials including p-type GaN are described in U.S. Pat. No. 6,472,300 to Nikolaev et al., the contents of which are incorporated herein by reference, and WO 00/68470 to Solomon et al. While such system have been used with some effectiveness in the past, they may not provide stable p-type doping during long growth processes due to surface degradation and the changes in the size of the metallic acceptor sources (e.g., magnesium and/or zinc).
For example, a HVPE system described by Solomon et al. utilizes metallic magnesium a source of magnesium dopant. The metallic magnesium is housed within a dopant chamber that is positioned outside of a reactor or growth tube. The metallic magnesium is heated, and hydrogen carrier gas is used to deliver magnesium to the reactor and form p-type GaN. Solomon et al. also describe another HVPE system that involves passing HCl gas over a group III metal/Mg mixture to form a first reagent gas, which reacts with ammonia to form p-type GaN.
HVPE systems that utilize a metallic magnesium sources, however, are not suitable for bulk growth or long-duration growth cycles due to degradation of the metallic magnesium source which, in turn, results in inconsistent doping and doping characteristics that are not readily reproducible. Further, Solomon et al. describe using hydrogen as a carrier gas for the metallic magnesium. Hydrogen gas, however, is not desirable for this purpose due to the resulting high electrical resistivity of the grown gallium nitride material. Moreover, such systems are associated with accumulation of magnesium-containing compounds on the inner surfaces of gas delivery tubes of the reactor. Additionally, metallic magnesium have to be close to or above its melting point for purposes of high level doping of grown materials, but the resulting melted magnesium may react with boats or containers holding source materials, thereby contaminating internal components and source materials and resulting in inconsistent and unstable doping. Further, while it is known to use Cp2Mg in MOCVD reactors, such materials have not been utilized in HVPE reactors due to, for example, potential decomposition of Cp2Mg in HVPE reactor before magnesium-containing gases reach the growth zone, deposition of magnesium-containing compounds on internal reactor components and low and uncontrollable doping levels.